What do we talk about when we talk about rhythm?

Neural oscillations align to external stimulus rhythms, such as the recurring onsets in rhythmic sequences, via neural entrainment-that is, adjustment to the oscillation's phase and period.As such, measures of neural phase coherence or "phase concentration" can inform us about the precision of entrainment.Entrainment of ongoing neural oscillations is considered a potent mechanism that the brain utilizes to generate temporal predictions and to aid active perception, at least in sensory cortices.However, as attempts to falsify or question this hypothesis are scarce, the idea that temporal predictions are instantiated in neural oscillatory entrainment should not be set in stone just yet.For this reason, it is very important that a recent study by Breska and Deouell, published in PLOS Biology [1], set out to determine whether phase concentration at the time of a critical target stimulus would also be observed in instances when stimulus sequences are non-rhythmic, but targets are nonetheless temporally predictable [2].Indeed, temporal predictions can certainly be formed on the basis of non-rhythmic information, such as the passage of time itself [3].Breska and Deouell propose a "memory-based" prediction mechanism that would not be instrumented by entrainment but instead might utilize alternate, interval-based timing mechanisms [4,5].While the study raises very important questions, logical and methodological considerations have motivated us to lay out a short guide for future experiments that aim to demonstrate oscillatory entrainment or the lack thereof.Throughout, we support our arguments with simulations and supplemental audio, which can be found at https://osf.io/phs6e/.Breska and Deouell aimed to assess behavioral and electrophysiological measures in contexts that vary in their rhythmicity, with the goal of exploring the bounding conditions of neural entrainment as well as the neural mechanisms that might support temporal predictions when entrainment is not possible.The trouble with examining neural mechanisms in contexts with varying rhythmicity is that we are still lacking a good working definition of rhythm.What does "rhythm" mean to a human or non-human brain, and to a perceiver more generally, and how variable does a sequence of events need to be so that our brains will cease to register it as rhythmic?Many researchers seem to use "rhythm" to refer to isochrony, that is, strict regularity, and contrast this with situations in which sequences are composed using variable stimulus onset asynchronies (SOAs; e.g., "repeated-interval" and "random" conditions in Breska and Deouell's study).However, there are many types of sequences that might be perceived as rhythmic despite being decidedly aperiodic and having relatively high variance in terms of the intervals making up the rhythm.For example, metrical musical rhythms comprising intervals of different sizes

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